bash-completion: add completion for `ostree sign`
bash-completion: add completion for `ostree sign`

Add bash completion with supported options for signing command.

Signed-off-by: Denis Pynkin <3fbf95ee68899518ee8799d35f88c8f7982aa936@collabora.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree